Pass opencl_platform_index to the GCMC sampler

CHANGELOG.md

@@ -34,6 +34,7 @@ Changelog
 * Fixed alchemical ions changing their charge linearly over the whole lambda range for the `annihilate`, `decouple`, `charge_scaled_morph`, `ring_break_morph` and `reverse_ring_break_morph` lambda schedules, rather than alongside the perturbable molecule. A warning is raised for custom lambda schedules, where alchemical ions still use the `standard_morph` schedule [#263](https://github.com/OpenBioSim/somd2/pull/263).
 * Add a `ghostly_kwargs` option to pass options to `ghostly` when applying ghost atom modifications, e.g. `--ghostly-kwargs k_soft=3 stiffen_rotamers=true` on the command line. Options are validated against those supported by `ghostly`. Systems that have already been modified with `ghostly` raise an error unless `ghost_modifications` is disabled, or `force=True` is passed in `ghostly_kwargs` [#264](https://github.com/OpenBioSim/somd2/pull/264).
 * Allow `equilibration_time` to be a list of times, one for each lambda window, e.g. `--equilibration-time 0ps 100ps 100ps 0ps` on the command line. Windows with a time of zero are not equilibrated. A single time is still used for every window [#265](https://github.com/OpenBioSim/somd2/pull/265).
+* Pass `opencl_platform_index` to the GCMC sampler, so that GCMC runs on the same OpenCL device as the dynamics when there are multiple OpenCL platforms [#266](https://github.com/OpenBioSim/somd2/pull/266).
 
 [2026.2.0](https://github.com/openbiosim/somd2/compare/2026.1.0...2026.2.0) - Sep 2026
 --------------------------------------------------------------------------------------

src/somd2/runner/_base.py

@@ -1185,6 +1185,10 @@ def _set_dynamics_kwargs(self):
                 "batch_size": self._config.gcmc_batch_size,
                 "num_attempts": self._config.gcmc_num_attempts,
             }
+            if self._config.platform == "opencl":
+                self._gcmc_kwargs["opencl_platform_index"] = (
+                    self._config.opencl_platform_index
+                )
         else:
             self._gcmc_kwargs = None